fix(avatars): load profile image on settings and sign in pages if available

Fixes #1727.
fix(avatars): ensure the default image background is covered by the profile image
refactor(avatars): remove Session cache and load avatar from server
refactor(Session): use User and Account models in lieu of Session

This refactor changes a number of things:
- introduces User and Account models
- imports old Session based sessions into User
- handles direct link to settings page from RPs for a specified account (fixes #1686)
- redirects unverified accounts that visit /settings (fixes #1716)
- introduces a way to pass ephemeral data between views
- removes unused avatar-change-via-URL resources/tests (fixes #1544, fixes #1618)
refactor(account): User always returns an account; no more null
refactor(views): use private var notation and othe clean-up
tests(settings): more functional tests coverage of settings page

Adds tests for issue #1716 and #1686.
refactor(account): update fx-desktop broker to work with user and account models
tests(change_password): remove unnecessary unverified account checks
refactor(accounts): only try a profile request if access token is present
fix(accounts): only fetch access token if verified
chore(convict): update convict
fix(broker): fix desktop broker to forward account data before confirm
Merge pull request #1733 from mozilla/issue-1727-load-avatar-on-profile

fix(avatars): load profile image on settings and sign in pages if available

Fixes #1727, #1722, #1686, #1716, #1544, #1618
r=ckarlof,shane-tomlinson